Method for reconditioning bacteria-contaminated hydrogen sulfide removing systems

ABSTRACT

A method is provided for reconditioning a hydrogen sulfide removal system, such as a Stretford process system in which elemental sulfur is formed, after the system has become contaminated with sulfur-feeding bacteria to an extent that operation thereof is substantially impaired. The method includes adding a bactericide to the washing solution employed in the system to destroy the bacteria and adding a surfactant to the solution to remove dead bacteria from the surfaces of particulate sulfur formed in the system so that the particles can agglomerate in the intended manner to enable sulfur removal from the solution.

RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 331,070 filedDec. 16, 1981 (now U.S. Pat. No. 4,393,037), and of application Ser. No.441,669 filed Nov. 15, 1982 (now abandoned), which in turn was acontinuation-in-part of application Ser. No. 331,070 filed Dec. 16,1981.

BACKGROUND OF THE INVENTION

The present invention relates generally to the removal of hydrogensulfide from gas streams by contacting a gas stream with an aqueouswashing solution, and more particularly to reconditioning of suchsolutions and associated hydrogen sulfide removal systems after theyhave become contaminated with sulfur-feeding bacteria which thrive inthe washing solution and which adversely affect removal of hydrogensulfide by the system.

With the increasing concern over atmospheric pollution and theconcomitant increasingly strict enforcement of tight air pollutionstandards, ever greater responsibility is placed on industry to producepollution-free products in a non-polluting manner. An area of particularenvironmental concern has been, and is, the discharge or release ofsulfur and its compounds, especially hydrogen sulfide, into theatmosphere as a result of various industrial processes. These processesinclude, for example, petroleum refining, the sweetening of sour naturalgas, destructive distillation of coal and oil shale, gasification orliquefication of coal and production and use of hydrogensulfide-containing geothermal steam and liquid for generatingelectricity or for other uses.

Several processes have been developed, and are in relatively common use,for removing hydrogen sulfide from gas streams such as those generatedand/or encountered in the industrial processes listed above. Suchhydrogen sulfide removal processes as the Stretford and the Takahaxprocesses, employ an aqueous, alkaline washing solution to absorbhydrogen sulfide from a gas stream, for example, a tail gas from a Clausapparatus. Vanadates in the washing solution convert the hydrogensulfide to elemental sulfur, which is then removed in particulate formfrom the washing solution. The sulfur removal may be accomplished in anautoclave in which small sulfur particles are agglomerated for removalpurposes. Washing solutions employed in these hydrogen sulfide removalprocesses are typically regenerated for reuse by injecting air into andthrough the solutions, the oxygen contained in the air reoxidizing thevanadate or other metal compound used.

By way of background, the washing solution employed in the Stretfordprocess contains a water-soluble salt of 9,10 anthraquinone disulfonicacid (ADA) and a water-soluble vanadate (or other multivalent metal)compound. The Takahax process, in contrast, uses a washing solutioncontaining a water-soluble naphthaquinone sulfonate (NQS), alone or incombination with a multivalent metal compound. The hydrogen sulfideremoval process disclosed in U.S. Pat. No. 4,283,379 to Fenton et al.employs a washing solution containing solubilized vanadium, one or morewater-soluble non-quinone aromatic compounds, thiocyanate ions and awater-soluble carboxylate complexing agent.

It is known in the petroleum industry that certain types of bacteriawhich live on sulfur can cause serious problems in those petroleumoperations in which sulfur and/or sulfur compounds are present. U.S.Pat. No. 3,329,610 to Kreuz et al., for example, discusses the bacterialproblems encountered in secondary oil recovery operations employingwater flooding or injection. Sulfur-feeding bacteria in these secondaryoil recovery operations have been found to cause severe corrosion ofmetal equipment and, when present in sufficiently large numbers, tocause plugging of pores in the associated oil bearing earth formations.The two classes of bacteria believed responsible for these problems areidentified in the Kreuz et al. patent as Disulforibrio desulfuricans(sulfate reducers), an anerobe, and Pseudomonas, an aerobe.

Recently, applicants have discovered sulfur-feeding bacteria in certainhydrogen sulfide gas removal systems, namely Stretford process systems.Although some hydrogen sulfide washing solutions, such as thosedisclosed in the above-cited patent to Fenton et al., are consideredhostile to living organisms because they contain thiocyanate compounds,other hydrogen sulfide washing solutions, such as those used in theStretford process, appear to support the growth of at least some typesof sulfur-feeding bacteria. The Stretford process washing solutions,because of their hot, aqueous nature, appear to promote the breeding ofsulfur-feeding bacteria to an extent that severe problems with hydrogensulfide removal may result.

In specific respect to the observed sulfur bacteria problems inStretford process washing solutions, bacteria entering the elementalsulfur-removing autoclave are apparently killed by the increasedtemperature encountered in the autoclave. After being killed, however,the dead bacteria have been found to remain adhered to the sulfurparticles on which they had been feeding and thereby form a coatingwhich prevents the sulfur particles from agglomerating in the normalmanner required for sulfur removal. As a consequence, the sulfurparticles are recirculated with the washing solution from the autoclaveback into the rest of the system and interfere with the hydrogen sulfidegas removal process.

The living bacteria in the system, particularly in the solutionregeneration (vanadate reoxidation) vessel, have been found to producelarge amounts of slime and to cause excessive solution frothing, botheffects inhibiting the solution regeneration process. Since elementalsulfur is formed in the regeneration vessel, the vessel is considered agood breeding ground for the sulfur-feeding bacteria, the bacteria beingbred in the vessel at a faster rate than bacteria are killed in theautoclave. Since the washing solution is also recirculated from theregeneration vessel, sulfur-feeding bacteria are continually circulatedthrough the entire system from the vessel.

Due to the effects of the sulfur-feeding bacteria, the hydrogen sulfideremoval capacity of the washing solution rapidly decreases as thebacterial contamination increases. As a result, the industrial processwith which the hydrogen sulfide removal process is associated mustusually be curtailed with production being lost and/or production costsbeing increased.

Until recently, degradation of hydrogen sulfide washing solutions hasnot been attributed to sulfur-feeding bacteria. Most attempts toalleviate problems of washing solution degradation are believed to haveinvolved replacing only a portion of the degraded washing solution withfresh washing solution. However, in the case of bacterial contaminationof the washing solution, replacement of only a portion of the washingsolution would result in bacterial contamination of the newly addedsolution, and any beneficial effects of adding the fresh solution wouldbe only temporary.

Since hydrogen sulfide removal systems, such as Stretford processsystems, typically employ between about 100,000 and 500,000 gallons ofwashing solution, total washing solution replacement is relativelycostly. Substantial additional costs may be associated with disposal ofthe replaced washing solution. As a result, replacement of the entirewashing solution is not economically attractive, even if it werepossible to clean the system sufficiently to remove all traces ofbacteria which could cause infestation of the replacement washingsolution. Also, considerable down time would be required for thecleaning operation.

In respect to known sulfur feeding bacterial problems associated withwater flooding or injection for secondary oil recovery, theabove-identified patent to Kreuz et al., as well as U.S. Pat. Nos.2,987,475 and 3,049,492 to Legator and DeGroote et al., respectively,disclose use of specific bactericides with which the water may betreated as a preventative measure. Use of peracetic acid, olefinicallyunsaturated lower alkyl aldehydes and certain oxirane ring containingcompounds as bactericides are disclosed in the patents mentioned above.However, none of these disclosed bactericides are considered adaptablefor use in hydrogen sulfide washing solutions, because none of thesebactericides is chemically stable at the conditions existant withintypical hydrogen sulfide washing solutions. In such washing solutionseach of these bactericides rapidly react with chemical constituentspresent in the washing solution, thereby decomposing into lessbactericidally active materials. Peracetic acid, for example, reactswith hydrogen sulfide and the reduced form of the multivalent metal ion(V⁺⁴ in Stretford solutions) to form relatively bactericidally inactiveacetic acid. Olefinically unsaturated lower alkyl aldehydes react withgaseous oxygen to form bacteriocidally active peracids but, as is thecase with peracetic acid, these peracids react with hydrogen sulfide andthe reduced form of the multivalent metal to form relativelybactericidally inactive organic acids. Oxirane rings react with water toform relatively bactericidally inactive glycols.

A significant aspect of the problem of bacterial contamination orinfestation of hydrogen sulfide removal systems, such as Stretfordprocess systems, is that the contamination problem is not universal butseems to occur only in a portion of the hydrogen sulfide removalsystems. Although the bacterial contaminations appear most often insystems located in hot, moist climates, as may be found, for example,around the Gulf of Mexico in the United States, not all similar systemsin this type climate become contaminated. Furthermore, the bacterialinfestations appear to be cyclic in nature and may be dependent upon asyet unknown cyclic characteristics of the bacteria. As a result, evenafter the bacterial nature of the hydrogen sulfide removal process hasbeen identified, universally applied preventive techniques do not appeareconomically feasible.

An important need therefore exists for an effective economical methodfor reconditioning hydrogen sulfide removal systems which have becomeinfested or contaminated by sulfur-feeding bacteria to an extent thateffectiveness of the hydrogen sulfide removal system is substantiallyimpaired.

Accordingly, it is a principal object of the present invention toprovide a method for reconditioning hydrogen sulfide removal systems andwashing solutions which have become badly infested with sulfur-feedingbacteria.

Still another object of the invention is to prevent subsequent bacterialreinfestation of the hydrogen sulfide removal system and washingsolution by the sulfur-feeding bacteria after the system and solutionhave been reconditioned.

Further objects, advantages and features of the invention will becomeapparent to those skilled in the art from the following description whentaken in conjunction with the accompanying drawing.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method is provided forreconditioning a hydrogen sulfide gas removal system utilizing anaqueous washing solution in which hydrogen sulfide is reacted to formparticles of elemental sulfur, especially in systems employing means foragglomerating such particles to enable sulfur removal from the washingsolution and means for regenerating the washing solution to enable itsreuse when the system and the washing solution become contaminated withliving, sulfur-feeding bacteria to an extent that normal operation ofthe sulfur removal system and washing solution regenerating means aresubstantially impaired. The reconditioning method comprises contactingthe sulfur-feeding bacteria in the solution with a suitable bactericidalagent in an amount destroying substantially all of the bacteria. Themethod further includes adding to the solution a suitable surfactant inan amount to remove a sufficient number of destroyed bacteria fromsurfaces of the sulfur particles to enable agglomeration of the sulfurparticles for removal purposes.

The method of this invention reconditions a hydrogen sulfide removalsystem and washing solution employed therein by destroyingsulfur-feeding bacteria contaminating the system and solution and bypreventing the destroyed bacteria, as well as any bacteria killed in thesulfur removing means, from adhering to surfaces of sulfur particlesformed in the solution during the hydrogen sulfide removal process. Themethod also prevents recontamination of the system and washing solutionby unkilled bacteria left in the system or solution or by live bacteriaentering the system or solution from the outside.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be more readily understood by reference tothe drawing, in which is depicted an exemplary hydrogen sulfide gasremoval system with which the present recondition-method may be used toadvantage.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The reconditioning method of the present invention is suitable for usewith a wide variety of hydrogen sulfide removal systems and the washingsolutions employed therein when such systems and washing solutions havebecome severely contaminated or infected with sulfur-feeding bacteria.Although, for illustrative purposes, the method is hereinbelow describedin particular respect to reconditioning a Stretford process systememploying an aqueous, alkaline washing solution containing awater-soluble vanadate and water-soluble anthraquinone disulfonic acid(ADA), it is to be understood that the method is not so limited but isalso applicable to other hydrogen sulfide removal systems employingwashing solutions which promote the growth of sulfur-feeding bacteria.

The term "sulfur-feeding bacteria" is used herein for convenience and isto be attributed a broad meaning so as to include bacteria or livingmicroorganisms of any type that act detrimentally on or with sulfur,regardless of the particular mechanism involved. For example, asulfur-carbon transfer is believed associated with at least some typesof bacteria which are considered to be sulfur-feeding within the broaddefinition of the term. Carbon from the solution, presumably fromcarbonates, is ultimately absorbed by these bacteria for cell buildingneeds. In the transfer process, sulfur is converted to sulfates, whichmay additionally inhibit the hydrogen sulfide removal capacity of thesystem.

For a better understanding of the invention, the drawing depicts anexemplary Stretford process system, into which, in accordance with theinvention, a bactericide and a surfactant are added after the system hasbecome contaminated with sulfur-feeding bacteria. Shown generallycomprising the system are absorber vessel 12, balance tank 14, reactiontank 16, oxidizer vessel 18 and autoclave 20. Also included in system 10are mixing vessel 22, froth tank 24 and air supply 26.

A hydrogen sulfide-containing gas stream, for example a Claus processtail gas stream, flows into lower regions of absorber vessel 12 throughconduit 38. Typically, the gas stream contains between about 0.5 andabout 5.0 mole percent of hydrogen sulfide. From balance tank 14,hydrogen sulfide washing solution flows into upper regions of absorbervessel 12 through conduit 40. A pump, not shown, may be used to pump thewashing solution from tank 14 into vessel 12.

Within vessel 12, which may contain a number and/or variety of baffleplates, not shown, the hydrogen sulfide gas-containing stream fromconduit 38 rises towards the top of vessel 12 in counter-flow with thedownwardly flowing washing solution introduced from conduit 40. As aresult of this counter-flow, and preferably intermixed flow, of gas andwashing solution through vessel 12, hydrogen sulfide is extracted fromthe gas by the washing solution and is carried along therewith.Alternatively, one or more venturi or other gas-liquid contactingapparatus may be employed in conjunction with, or in replacement of,absorber 12. Sodium carbonate may be utilized in the washing solution toreact with the hydrogen sulfide from the gas stream in vessel 12 to formsodium hydrosulfide according to the following reaction:

    H.sub.2 S+Na.sub.2 CO.sub.3 →NaHS+NaHCO.sub.3       (1)

In turn, the sodium hydrosulfide is oxidized to elemental sulfur by thewater-soluble vanadate in the solution according to the reaction:

    NaHS+NaHCO.sub.3 +2NaVO.sub.3 →S+Na.sub.2 V.sub.2 O.sub.5 +Na.sub.2 CO.sub.3 +H.sub.2 O                                       (2)

The hydrogen sulfide stripped gas is vented from the top absorber vessel12 through conduit 42, venting being normally into the atmosphere. Thewashing solution is discharged from the bottom of vessel 12 throughconduit 44 into reaction tank 16 where further and complete chemicalconversion to elemental sulfur, in accordance with reaction (2), occurs.The washing solution and gas stream residence time in vessel 12typically varies between about 10 and 15 minutes.

The sulfur-containing washing solution flows from reaction tank 16through conduit 46 into the lower regions of oxidizer vessel 18. A pump(not shown) is used to pump the solution from tank 16 into oxidizervessel 18. Air is injected into the bottom of oxidizer vessel 18 fromair supply 26, through conduit 48, and is preferably churned into alarge number of small bubbles by rotating stirrer-mixer 50 located abovethe air inlet. These air bubbles rise through the solution in oxidizervessel 18, thereby reoxidizing the solution, which was chemicallyreduced in absorber vessel 12 and reaction tank 16, to its initialstate. Thus, the vanadium, which by reaction (2) had been reduced fromits pentavalent state to the quadravalent state (V⁺⁵ to V⁺⁴), isoxidized in oxidizer vessel 18 by the upwardly flowing air from source26, back to its original pentavalent state, the ADA in the washingsolution acting as a catalyst. The reaction involved is: ##STR1##

In this manner, the washing solution is regenerated in oxidizer vessel18 for reuse in the system.

Most of the elemental sulfur in oxidizer vessel 18 rises to the top ofthe washing solution, identified generally as 62, and forms a froth 64on top of solution 62. This sulfur froth 64 is withdrawn, accompanied bya small amount of regenerated washing solution, through conduit 66 tofroth tank 24. Some of the sulfur in oxidizer vessel 18, however,settles to the bottom of the vessel to form sulfur layer 68. Thereconditioned washing solution with the exception of that amountdischarged with sulfur froth 64 into conduit 66, is discharged throughconduit 70 to mixing vessel 22.

From froth tank 24 the elemental sulfur and washing solution aredischarged through conduit 72 into autoclave 20. The sulfur, dischargedinto autoclave 20 as small particles, is agglomerated at an elevatedtemperature and then drawn off from bottom regions of autoclave 20through conduit 74. The washing solution is recirculated from autoclave20 through conduit 76, which is connected into conduit 70 betweenoxidizer vessel 18 and mixing vessel 22.

Make-up chemicals, such as ADA and vanadate, are added to the washingsolution in mixing vessel 22 via conduit 78 in amounts required tomaintain the desired concentration levels in the washing solution. Frommixing vessel 22, the washing solution, which is largely regeneratedsolution from oxidizer vessel 18, is discharged through conduit 79 tobalance tank 14. As previously described, the washing solution frombalance tank 14 is discharged into absorber vessel 12 through conduit40, thereby completing the washing solution recirculation loop.

When the system becomes contaminated or infested with sulfur-feedingbacteria, the bacteria tend to become concentrated in sulfur froth 64 inoxidizer vessel 18 and are discharged with the sulfur through froth tank24 into autoclave 20, wherein most of the bacteria are killed at thehigher autoclave temperature. The dead bacteria, however, adhere orremain attached to the sulfur particle surfaces and thereby prevent theagglomeration of the particles required for sulfur removal. As a result,the sulfur particles remain in suspension in the washing solution andare recirculated from autoclave 20 with the washing solution throughconduit 76.

The sulfur layer 68 at the bottom of oxidizer vessel 18 also provides arich breeding ground for the bacteria, and indeed, bacteria in thesulfur layer tend to reproduce at a faster rate than that at whichbacteria are removed with sulfur froth 64. As a consequence, thebacteria population in the system increases rapidly after intitialsystem contamination by the bacteria.

Apparently as the result of the bacteria breeding in oxidizer vessel 18,in particular in sulfur layer 68, an excessive amount of sulfur frothingoccurs and large amounts of slime are formed in oxidizer vessel 18. Boththe excessive sulfur frothing and slime formation inhibit the solutionregeneration (vanadium reoxidation) reaction in oxidizer vessel 18,thereby degrading the effectiveness of the washing solution. Inaddition, some of the excessive sulfur froth and slime formed in theupper regions of oxidizer vessel 18 are recirculated with the washingsolution back upstream through mixing vessel 22 and balance tank 14 intoabsorber vessel 12 and therein inhibit the hydrogen sulfide removalprocess.

As a result of bacterial growth and increasing bacterial contamination,the effectiveness of the washing solution for removing hydrogen sulfidefrom the gas stream entering absorber vessel 12 through conduit 38diminishes, often at a quite rapid rate. This degradation of the systemin turn requires curtailment of the associated industrial process fromwhich the hydrogen sulfide-containing gas stream is received in orderthat allowable hydrogen sulfide emission levels not be exceeded.

The present method reconditions the washing solution and the entiresystem during operation and the system is returned to normal operationwithout necessity for washing solution replacement.

Accordingly, the reconditioning method comprises adding a bactericide orbactericidal agent into the bacteria-contaminated washing solution in anamount and at a rate destroying all or substantially all of thesulfur-feeding bacteria in the entire system. As shown in the FIGURE,source 86 of the bactericidal agent discharges into mixing vessel 22through conduit 88 so as to enable intimate mixing of the bactericidalagent with the washing solution. Valve 90 in conduit 88 enables controlof the amount of bactericidal agent added from source 86 into mixingvessel 22. Less preferably, source 86 may alternatively be flowconnected into balance tank 14 or into oxidizer vessel 18.

The normal recirculating flow of washing solution from mixing vessel 22rapidly and thoroughly distributes the bactericidal agent throughout thesystem, thereby causing rapid destruction of the sulfur-feedingbacteria. As above-mentioned, however, the dead bacteria tend to clingto the surface of sulfur particles on which they were feeding and soinhibit sulfur particle agglomeration in, and hence sulfur removal from,autoclave 20. In consequence, the sulfur and dead bacteria arerecirculated through the system from autoclave 20 so as to inhibit thehydrogen sulfide removal process. The excessive sulfur froth and slimeformed in oxidizer vessel 18 by the bacteria are circulated through thesystem from vessel 18 and also inhibit the hydrogen sulfide removalprocess. Thus, although addition of the bactericidal agent to mixingvessel 22 prevents further degradation of the hydrogen sulfide removalprocess in the system, the bactericidal agent does not by itselfrecondition the system to its pre-bacterial contamination effectiveness.

Therefore, to fully recondition the system after bacterialcontamination, a surfactant is also added. As shown in the Drawing,source 92 of surfactant is flow connected into mixing vessel 22 byconduit 94. Valve 96 in conduit 94 controls flow of surfactant fromsource 92 into mixing vessel 22. Surfactant from source 92 mayalternatively, but less preferably, be added into balance tank 14 oroxidizer vessel 18.

As the surfactant is circulated in the washing solution through thesystem from mixing vessel 22, the recirculated sulfur particles, deadbacteria, froth and slime are swept into autoclave 20 from which theyare removed, through conduit 74, by the normal sulfur removal process.Accordingly, much larger than normal amounts of sulfur, with the othercontaminants, are removed at first and effective system reconditioningis evidenced by unusually large amounts of sulfur being extracted fromautoclave 20.

Several days of operation are usually required, after addition of thebactericidal agent and surfactant into mixing vessel 22, before thesystem is completely reconditioned. During these several days, hydrogensulfide removal capacity is gradually restored and effectiveness of thesystem is increased so that any curtailment of the associated industrialprocess can be reduced. It is emphasized that this system reconditioningis normally accomplished without necessity for replacement of any of thewashing solution, although recovery of the system may in some instancesbe accelerated by accompanying replacement of some of the washingsolution.

After the system has been completely reconditioned by addition ofsufficient amounts of the bactericidal agent and surfactant, furtheraddition of bactericidal agent, at least for a period of time assuringthat no recontamination occurs by sulfur-feeding bacteria which may haveintitially escaped destruction, may be desirable. Consequently, thepresent method optionally includes introduction of additionalbactericidal agent from source 86 into the system, preferably intomixing vessel 22. Ordinarily, the rate at which the bactericidal agentis added after the system has been completely reconditioned is less thanthe rate at which the bactericidal agent is added to achieve systemreconditioning. That is, the concentration of bactericidalrecontamination of the system will normally be less than that requiredto recondition the system, particularly when relatively large amounts ofbactericidal agent have been used to achieve rapid systemreconditioning.

It will be appreciated that the type and amounts of bactericidal agentand surfactant required to recondition the system depend upon suchfactors as the extent of bacterial contamination, the length of time thesystem has been contaminated, the type of bacteria involved, and thetype of hydrogen sulfide removal system.

The bactericidal agent used in this invention comprises a compoundselected from the group of bactericidal agents which are adaptable tosulfur removal systems employing an aqueous washing solution. Such groupconsists of bactericidal agents which

(1) are sufficiently water soluble to allow the dissolving of abacterially-lethal concentration of such agent into the aqueous washingsolution;

(2) are substantially stable at the conditions existant within thewashing solution;

(3) are substantially innocuous to the oxidation of hydrogen sulfide tosulfur;

(4) are substantially innocuous to the regeneration of the washingsolution with oxygen;

(5) are substantially innocuous to the recovery of product sulfur;

(6) are substantially nonreactive or decomposable within the washingsolution except in cases where essentially all products of such reactionand/or decomposition are substantially innocuous to the environment andsubstantially innocuous to the hydrogen sulfide removal system.

Preferably, the bactericidal agent used in this invention can be storedand used without being unduly dangerous to human beings. As used herein,the phrase "innocuous to the environment" means: will not cause orsignificantly contribute to air pollution problems or to liquid wastedisposal problems. The phrase "innocuous to the hydrogen sulfideremoving system," as used herein, means: will not interfere with theoxidation of hydrogen sulfide to sulfur, will not interfere with theregeneration of the washing solution with oxygen, will not interferewith the recovery of product sulfur, and will not in practice accumulatewithin the washing solution to the point where such accumulationinterferes with the oxidation of hydrogen sulfide, the regeneration ofthe washing solution or the recovery of product sulfur.

The bactericidal agents suitable for use in this invention includetetrahydro-3,5 dimethyl-2H-1,3,5-thiadiazine-2-thione, 2-(4-thiazolyl)benzimidazole, p-chloro-m-cresol, sodium pyridinethione-N-oxide, sodiumtetraborate pentahydrate, tributyltin oxide, tributyltin fluoride, bistributyltin oxide, parachlorometacresol, methylenebisthiocyanate,2,3,5,6 tetrachloro-4-(methyl sulfonyl)-pyridine,3,4,5-trichloro-2,6-dicyanopyridine, sodium pyridinethione-N-oxide,alkyl dimethyl ethylbenzyl ammonium cyclohexylsulfamate andbenzisothiazolone, as well as many alkyl tin oxides, alkyl tinfluorides, thiones, thiocyanates, sulfonicpyridines, cyano-pyridines,quaternary ammonium cations and amino alcohols.

The bactericidal agents preferred for use in this invention are thewater-soluble phenolic compounds including unsubstituted phenols andsubstituted phenols including their salts and their hydrate forms, withphenol being the most preferred bactericidal agent. As used herein, theterm "phenol" means the organic compound consisting of a benzene ringhaving one hydroxyl group joined to the ring.

The unsubstituted phenols which can be used include phenol and variouswater-soluble phenol salts. Exemplary water-soluble, unsubstitutedphenols include phenol and sodium phenoxide.

The substituted phenols that can be used include various water-solublehydroxy-substituted phenols, carboxy-substituted phenols,sulfonate-substituted phenols, amino-substituted phenols,amino-substituted phenols, nitro-substituted phenols, and their phenolsalts. Other substituted phenols which can be used include variouswater-soluble alkyl-substituted phenols, aryl-substituted phenols,benzo-substituted phenols, halogen-substituted phenols and their phenolsalts. Exemplary water-soluble, substituted phenols includehydroquinone; resorcinol; catechol; mixtures of hydroquinone, catechol,and resorcinol; salicylic acid; 3,5-disulfopyrocatechol; p-aminophenol;p-hydroxybenzamide; p-nitrophenol; ortho-cresol; meta-cresol;para-cresol; cresol (commercial mixture of o, m, and p-cresol);p-carboxyphenol; 1-nitro-2-naphthol; 1-naphthol; and chlorophenols.

If salts of the water-soluble, phenolic complexing agents are used, thesodium salt is preferred, although salts of other alkali metals, such aspotassium, can also be used.

As used herein, the term "substituted" is not exclusive and allows forsubstitutions other than the group denominated. For example, acarboxy-substituted phenol may have one or more carboxy groups and oneor more other groups substituted thereon.

Normally the bactericidal agent is added to the washing solution at aconcentration of at least about 5 parts per million by weight (wppm) butgenerally below about 5,000 wppm, according to the extent of thebacterial infestation; a concentration range of between about 10 wppmand about 100 wppm being preferred, and between about 10 and about 50being more preferred. The range of about 10 wppm to about 100 wppm isparticularly adapted for the maintenance stage after the system andsolution have been reconditioned.

Like the bactericidal agent used in this invention, the surfactant usedin this invention must also be adaptable to sulfur removal systemsemploying an aqueous washing solution. Accordingly such surfactantcomprises a compound selected from the group of surfactants which:

(1) are, at least, partially water soluble;

(2) are substantially stable at the conditions existant within thewashing solution;

(3) are substantially innocuous to oxidation of hydrogen sulfide tosulfur;

(4) are substantially innocuous to regeneration of the washing solutionwith oxygen;

(5) are substantially innocuous to the recovery of product sulfur;

(6) are substantially nonreactive or decomposable within the washingsolution except in cases where essentially all products of such reactionand/or decomposition are substantially innocuous to the environment andsubstantially innocuous to said hydrogen sulfide removal system.

Preferably, the bactericidal agent used in this invention can be storedand used without being unduly dangerous to human beings. Because thebacterial coating effect on the sulfur which prevents sulfur particleagglomeration in autoclave 20 is non-ionic, a non-ionic surfactant ispreferred. However, ionic surfactants may alternatively be used,although, sometimes with less effectiveness.

Suitable surfactants include polyoxyethylene condensates represented bythe following general formula:

    R--O--(CH.sub.2 --CH.sub.2 --O).sub.n --H,                 (4)

wherein R is the residue of a fatty alcohol, acid, amide, or aminehaving from about 10 to about 18 carbon atoms or an alkyl phenol havingfrom about 10 to about 18 carbon atoms; and where n is an integer of 1or above and preferably between about 5 and about 30. Some specificexamples of polyoxyethylene condensates which can be used arepolyoxyethylene aliphatic ethers such as polyoxyethylene lauryl ether,polyoxyethylene oleyl ether, polyoxyethylene hydroabietyl ether and thelike; polyoxyethylene alkaryl ethers such as polyoxyethylene nonylphenylether, polyoxyethylene octylphenyl ether and the like; polyoxyethyleneesters of higher fatty acids such as polyoxyethylene laurate,polyoxyethylene oleate and the like as well as condensates of ethyleneoxide with resin acids and tall oil acids; polyoxyethylene amide andamine condensates such as N-polyoxyethylene lauramide andN-lauryl-N-polyoxyethylene thioethers such as polyoxyethylene n-dodecylthioether.

Some other examples of polyoxyethylene ether compounds arepolyoxyethylene nonylphenyl ether having a cloud point of between 126°and 133° F. and marketed under the trademark "Igepal CO-630" and apolyoxyethylene nonylphenol ether having a cloud point above 212° F. andmarketed under the trademark "Igepal CO-887." A similar polyoxyethylenenonyl-phenyl ether having a cloud point of about 86° F., marketed underthe trademark "Igepal CO-610," is also a good surfactant. "Igepals" aremarketed by General Analine and Film Company. Another surfactant is apolyoxyethylene actylphenyl ether having a cloud point of between 80° F.and 160° F. and marketed by Rohm and Haas Company under the trademark"Triton X-100." Other surfactants which may be used include apolyoxyethylene oleyl ether having a cloud point of between 80° F. and160° F. and marketed by ICI Americas, Inc. under the trademark "AtlasG-3915" and a polyoxyethylene lauryl ether having a cloud point above190° F. and marketed by Atlas Chemical Industries, Inc. under thetrademark "Brij 35."

The nonionic surfactants which can be used also include a group ofcompounds marketed by Wyandotte Chemicals Corporation under thetrademark "Pluronics." "Pluronics" have the following general formula:##STR2## wherein a, b and c are integers between about 1 and about 100.As the ratio of b to a and c increases, the compounds become less watersoluble and more oil soluble, and thus more hydrophobic; in contrast, asthe ratio of b to a and c decreases, the compounds become more watersoluble and less oil soluble, and thus more hydrophilic. An example ofthis group of compounds is "Pluronics L-61" which has a polyoxypropylenechain having a molecular weight of about 1,500 to 1,800 and apolyoxyethylene content that is about 5 to 15 percent of the totalweight of the molecule. Another example is "Pluronics L-64" which has apolyoxypropylene chain having a molecular weight of about 1,500 to 1,800and a polyoxyethylene content that is about 35 to 45 percent of thetotal weight of the molecule. Still another useful example is "PluronicsL-81" which has a polyoxypropylene chain having a molecular weight ofabout 2,100 to 2,600 and a polyoxyethylene content that is about 5 to 15percent of the total weight of the molecule. These compounds may beconsidered to be copolymers of polypropylene and polyethylene oxides.

Still another series of suitable surfactants that can be used areethylene oxide adducts of acetylenic glycols marketed by Air Products &Chemicals, Inc. under the trademark "Surfynol." "Surfynols" can berepresented by the following general formula: ##STR3## in which R₁ andR₄ are alkyl radicals containing from about three to about 10 carbonatoms, R₂ and R₃ are selected from the group consisting of methyl andethyl, and x and y are integers having a sum in the range of about 3 toabout 60.

Representative of the "Surfynols" are "Surfynol 365" which is anethylene oxide adduct of 2,4,7,9-tetramethyl decynediol containing anaverage of 10 moles of ethylene oxide per mole of the surfactant."Surfynol 485" corresponds to "Surfynol 465" but contains an average of30 moles of ethylene oxide per mole of surfactant. "Surfynol 485" has acloud point above 212° F.

Typically the surfactant is added to the washing solution at aconcentration of at least about 10 parts per million by volume (vppm),but generally below about 100,000 vppm, with a preferred concentrationrange of between about 1,000 vppm and about 5,000 vppm.

The present invention is further illustrated by the following exampleswhich are illustrative of various aspects of the invention and are notintended as limiting the scope of the invention as defined by theappended claims.

EXAMPLE 1

The following laboratory procedure is used to demonstrate theeffectiveness of the bactericidal agent and surfactant in reconditioninga Stretford washing solution.

Sulfur particles about 1/16 inch in diameter from an autoclave(corresponding to autoclave 20) are melted in an open beaker and arefound to have an apparent melting point of about 284° F., whereas themelting point of pure sulfur is about 235° F. A black frothy substanceis found at the top of the melted material and the melted materialitself is found to be black instead of yellow upon cooling. The meltedmaterial congeals at about 203° F. indicating a marked melting pointlowering due to impurities. After the black frothy material is removed,the material remelts at about 203° F.

An examination of the autoclave sulfur particles reveals that the moistsulfur cake from the froth tank is teeming with live bacteria ofcocci-type about 1 micron in diameter. The bacteria are vigorous inmovement, suggesting a phototropic nature, and are found to be feedingon the sulfur.

It is known that sulfur-feeding bacteria usually require temperatures ofabout 90° F. to survive and are killed by autoclave temperature.Cellulose, the dominant organic material in the bacteria cell wall, isknown to decompose upon heating to yield carbon. The melted sulfur isfound to contain about 5,000 wppm of carbon.

Some of the moist sulfur cake is filtered through an 8 micron filterpaper which permits the bacteria to pass through but not most of thesulfur. The sulfur is found to have a melting point of about 235° F.

Phenol is added to a solution containing the bacteria, at aconcentration level of about 10 wppm phenol; after 24 hoursapproximately 99 percent of the bacteria are found to have been killedby the phenol.

Approximately 100 milliliters of the nonagglomerated sulfur particleslurry from the autoclave are heated to about 284° F. for about 30minutes under 400 psig of nitrogen gas. Approximately 1,000 vppm of"Pluronic L-61" is added to the solution. When the solution is cooled,the sulfur particles are found to have agglomerated into a single largepiece of sulfur. Phenol alone is found to have little or no effect onsulfur agglomeration.

EXAMPLE 2

In a second experiment, a Stretford process system is sterilized byaddition of about 150 ppm of phenol-cresol mixture, with the bacteriakill being apparently complete. A sample of slurry from the system istreated with about 3,000 vppm of "Pluronic L-61" and all the sulfur isseparated as molten sulfur at temperature of either 260° F. or 280° F.

The entire Stretford process system is then treated with 250 vppm of"Pluronic L-61, and recovery of sulfur from the autoclave is abouttrebled. Very fine sulfur particles in the oxidizer vessel arecoagulated and separated by laboratory centrifuge. The removed sulfur isfound not to be of particularly good quality, whereupon more "PluronicL-61" is added to raise the total concentration thereof to about 1,000vppm. The quality of removed sulfur is improved by such addition. Soonafter the second addition of "Pluronic L-61," froth is found to collapsethroughout the system.

Although a particular embodiment of the invention has been described, itwill, of course, be understood that the invention is not limitedthereto, since many obvious modifications can be made, and it isintended to include within this invention any such modification as mayfall within the scope of the claims.

Having now described the invention, we claim:
 1. A method forreconditioning a hydrogen sulfide gas removal system employing anaqueous washing solution in which hydrogen sulfide is reacted to formelemental sulfur and which has means for agglomerating said particles toenable sulfur removal from said solution for reuse, after said systemand said solution have become contaminated with living, sulfur-feedingbacteria to an extent that normal operations of said sulfur removalmeans and of said solution regenerating means are substantiallyimpaired, said reconditioning method comprising the steps of:(a)contacting said sulfur-feeding bacteria in said solution with abactericidal agent to destroy substantially all of said sulfur-feedingbacteria; and (b) contacting said sulfur particles in said solution witha surfactant to remove a sufficient number of destroyed sulfur-feedingbacteria from surfaces of said sulfur particles to enable said particlesto agglomerate in said sulfur removal means;wherein said bactericidalagent is sufficiently water soluble to allow the dissolving of abacterially lethal concentration of said bactericidal agent into saidwashing solution, wherein said surfactant is nonionic and, at least,partially water soluble, and wherein both said bactericidal agent andsaid surfactant are substantially (1) stable at the conditions existantwithin said washing solution, (2) innocuous to the oxidation of hydrogensulfide to sulfur, (3) innocuous to the regeneration of said washingsolution with oxygen, (4) innocuous to the recovery of product sulfurand (5) chemically nonreactive or decomposable within the washingsolution except in cases where essentially all products of reactionand/or decomposition are substantially innocuous to the environment andsubstantially innocuous to the hydrogen sulfide removal system.
 2. Thereconditioning method defined in claim 1 wherein said surfactantcomprises a compound selected from the group consisting of:(a) apolyoxyethylene condensate represented by the following general formula:

    R--O--(CH.sub.2 --CH.sub.2 --O).sub.n --H,

wherein R is the residue of a fatty alcohol, acid, amide or amine havingfrom about 10 to about 18 carbon atoms or an alkyl phenol having fromabout 10 to about 18 carbon atoms and where n is an integer of 1 orabove; (b) a compound represented by the following general formula:##STR4## wherein a, b, and c are integers between about 1 and about 100;and (c) an ethylene oxide adduct of acetylenic glycols represented bythe general formula: ##STR5## wherein R₁ and R₄ are alkyl radicalscontaining from about 3 to about 10 carbon atoms, R₂ and R₃ are methylor ethyl and x and y are integers having a sum in the range of about 3to about
 60. 3. The reconditioning method defined in claim 1 whereinsaid surfactant is a copolymer of polypropylene and polyethylene oxides.4. The reconditioning method defined in claim 1 wherein saidbactericidal agent is added to said solution in an amount providing abacterial agent concentration level in said solution of between about 5wppm and about 5,000 wppm.
 5. The reconditioning method defined in claim1 wherein said bactericidal agent is added to said solution in an amountproviding a bactericidal agent concentration level in said solution ofbetween about 10 wppm and about 100 wppm.
 6. The reconditioning methoddefined in claim 1 wherein said surfactant is added to said solution inan amount providing a surfactant concentration in said solution ofbetween about 10 vppm and about 100,000 vppm.
 7. The reconditioningmethod defined in claim 1 wherein said surfactant is added to saidsolution in an amount providing a surfactant concentration in saidsolution of between about 1,000 vppm and about 5,000 vppm.
 8. Thereconditioning method defined in claim 1 further comprising the step,after said sulfur removing means has been reconditioned by said steps(a) and (b), of subsequently adding amounts of said bactericidal agentsufficient to prevent recontamination of said system by saidsulfur-feeding bacteria.
 9. The reconditioning method defined in claim 8wherein said additional bactericidal agent is added to said solution ata rate so as to maintain a concentration of said bactericidal agent insaid solution of between about 10 wppm and about 100 wppm.
 10. Thereconditioning method defined in claim 8 wherein said bactericidal agentconcentration is maintained between about 10 wppm and about 50 wppm. 11.A method for reconditioning a hydrogen sulfide gas removal systememploying an aqueous washing solution in which hydrogen sulfide isreacted to form elemental sulfur and which has means for agglomeratingsaid particles to enable sulfur removal from said solution and means forregenerating said solution for reuse, after said system and saidsolution have become contaminated with living, sulfur-feeding bacteriato an extent that normal operations of said sulfur removal means and ofsaid solution regenerating means are substantially impaired, saidreconditioning method comprising the steps of:(a) contacting saidsulfur-feeding bacteria in said solution with a bactericidal agent todestroy substantially all of said sulfur-feeding bacteria; and (b)contacting said sulfur particles in said solution with a surfactantadaptable to said hydrogen sulfide gas removal system to remove asufficient number of destroyed sulfur-feeding bacteria from surfaces ofsaid sulfur particles to enable said particles to agglomerate in saidsulfur removal means;wherein said bactericidal agent is comprised of acompound selected from the group consisting of tetrahydro-3,5dimethyl-2H-1,3,5-thiadiazine-2-thione, 2-(4-thiazolyl) benzimidazole,p-chloro-m-cresol, sodium pyridinethione-N-oxide, sodium tetraboratepentahydrate, tributyltin oxide, tributyltin fluoride, bis tributyltinoxide, parachlorometacresol, methylenebisthiocyanate, 2,3,5,6tetrachloro-4-(methyl sulfonyl)-pyridine,3,4,5-trichloro-2,6-dicyanopyridine, sodium pyridinethione-N-oxide,alkyl dimethyl ethylbenzyl ammonium cyclohexylsulfamate andbenzisothiazolone.
 12. The reconditioning method defined in claim 11wherein said bactericidal agent is sufficiently water soluble to allowthe dissolving of a bacterially lethal concentration of saidbactericidal agent into said washing solution, wherein said bactericidalagent is substantially (1) stable at the conditions existant within saidwashing solution, (2) innocuous to the oxidation of hydrogen sulfide tosulfur, (3) innocuous to the regeneration of said washing solution withoxygen, (4) innocuous to the recovery of product sulfur and (5)chemically nonreactive or decomposable within the washing solutionexcept in cases where essentially all products of reaction and/ordecomposition are substantially innocuous to the environment andsubstantially innocuous to the hydrogen sulfide removal system.
 13. Thereconditioning method defined in claim 11 wherein said bactericidalagent is added to said solution in an amount providing a bacterial agentconcentration level in said solution of between about 5 wppm and about5,000 wppm.
 14. The reconditioning method defined in claim 11 furthercomprising the step, after said sulfur removing means has beenreconditioned by said steps (a) and (b), of subsequently adding amountsof said bactericidal agent sufficient to prevent recontamination of saidsystem by said sulfur-feeding bacteria.
 15. The reconditioning methoddefined in claim 14 wherein said additional bactericidal agent is addedto said solution at a rate so as to maintain a concentration of saidbactericidal agent in said solution of between about 10 wppm and about100 wppm.
 16. A method for reconditioning a hydrogen sulfide gas removalsystem employing an aqueous washing solution in which hydrogen sulfideis reacted to form elemental sulfur and which has means foragglomerating said particles to enable sulfur removal from said solutionand means for regenerating said solution for reuse, after said systemand said solution have become contaminated with living, sulfur-feedingbacteria to an extent that normal operations of said sulfur removalmeans and of said solution regenerating means are substantiallyimpaired, said reconditioning method comprising the steps of:(a)contacting said sulfur-feeding bacteria in said solution with abactericidal agent to destroy substantially all of said sulfur-feedingbacteria; and (b) contacting said sulfur particles in said solution witha surfactant adaptable to said hydrogen sulfide gas removal system toremove a sufficient number of destroyed sulfur-feeding bacteria fromsurfaces of said sulfur particles to enable said particles toagglomerate in said sulfur removal means;wherein said bactericidal agentis sufficiently water soluble to allow the dissolving of a bacteriallylethal concentration of said bactericidal agent into said washingsolution, wherein said bactericidal agent is substantially (1) stable atthe conditions existant within said washing solution, (2) innocuous tothe oxidation of hydrogen sulfide to sulfur, (3) innocuous to theregeneration of said washing solution with oxygen, (4) innocuous to therecovery of product sulfur and (5) chemically nonreactive ordecomposable within the washing solution except in cases whereessentially all products of reaction and/or decomposition aresubstantially innocuous to the environment and substantially innocuousto the hydrogen sulfide removal system, and wherein said bacteriacidalagent comprises a compound selected from the group consisting of thealkyl tin oxides, alkyl tin fluorides, thiones, thiocyanates,sulfonic-pyridines, cyano-pyridines, quaternary ammonium cations andamino alcohols.
 17. The reconditioning method defined in claim 16wherein said bactericidal agent is added to said solution in an amountproviding a bacterial agent concentration level in said solution ofbetween about 5 wppm and about 5,000 wppm.
 18. The reconditioning methoddefined in claim 16 further comprising the step, after said sulfurremoving means has been reconditioned by said steps (a) and (b), ofsubsequently adding amount of said bactericidal agent sufficient toprevent recontamination of said system by said sulfur-feeding bacteria.19. The reconditioning method defined in claim 18 wherein saidadditional bactericidal agent is added to said solution at a rate so asto maintain a concentration of said bactericidal agent in said solutionof between about 10 wppm and about 100 wppm.